Published: Monday, 18 July 2016 03:02
Lake Tyrrell is a large ephemeral salt lake, the level of which is controlled by climate and groundwater. Up to a metre of water fills the basin during the wetter and cooler winter season, but evaporates during the summer, precipitating up to 10 cm of halite. Each year essentially the same pool of ions is redissolved by this annual freshening. The small percentage of gypsum precipated (< 2%) in the surface salt crust reflects the low calcium content of the brine which, in turn, is a function of the negligible net discharge of calcium from the groundwater system. The small influx of fine‐grained clastic sediment to the lake floor comes from surface runoff, wind, and reworking of older sediment from the shoreline.
The Lake Tyrrell basin lies in a setting in which three different groundwater types, identified by distinct salinities, interact with surface waters. A refluxing cycle that goes from discharging groundwater at the basin margin, to surface evaporation on the lake floor, to recharge through the floor of the lake, controls the major chemical characteristics of the basin. In this process, salts are leached downward from the lake floor to join a brine pool below the lake. This provides an outlet from the lake, especially under conditions that have been both drier and wetter than those of today. Enhanced discharge occurs under drier conditions, when the enclosing regional groundwater divide is lowered, whereas a rise in lake level increases the hydraulic head over that of the sub‐surface brine and promotes an increase in brine loss from the lake.
Sulphate‐reducing bacteria in a zone of black sulphide‐rich mud beneath the salt crust help prevent gypsum from being incorporated into the recent sedimentary record. However, below the upper 5 to 10 cm zone of bacterial activity, discoidal gypsum is being precipitated within the mud from the groundwater. These crystals have grown by displacing the mud and typically “float” in a clay matrix; in some zones, they form concentrations exceeding 50% of the sediment. The occasional laminae of more prismatic gypsum that occur within the upper metre of mud have crystallised from surface brines. The scarcity of these comparatively pure prismatic‐crystal concentrations probably is a function of unfavourable chemical conditions in the lake brine and of the role that sulphate‐reducing bacteria have played.
(James T. Tellera, J. M. Bowlerb & P. G. Macumberc ;Modern sedimentation and hydrology in Lake Tyrrell, Victoria pages 159-175 Journal of the Geological Society of Australia Volume 29, Issue 1-2, 1982 Published online: 01 Aug 2007)
Palaeomagnetic investigations by previous studies have demonstrated that Lake Tyrrell represents a remnant of a Pleistocene mega-lake, Lake Bungunnia. This mega-lake is known to have been relatively fresh, and as such is indicative of generally wetter climatic conditions in southeastern Australia during its existence. The drying of Lake Bungunnia commenced between 0.7 Ma and 1.2 Ma, and signaled the onset of aridity in southeastern Australia. Indications from this and other sites point to multiple cycles of wetting and drying, correlated to global glacial-interglacial cycles; as such, this period represents the greatest environmental transformation of the last 20 million years. However, due to periodic deflation resulting from cyclic wetting and drying of the lakebed, the sediment sequence covering this period at Lake Tyrrell is discontinuous. In addition, extensive oxidation of lakebed sediments occurs during dry phases, and wet phases are characterized by secondary mineral precipitation within existing sequences. These conditions have made conventional proxies for palaeo-environmental reconstruction difficult to apply. Microfossil and pollen preservation is generally poor, and sedimentary textures and mineral content altered in some sequences.
References, and further reading
Bowler JM (1976) Aridity in Australia: Age, Origins and Expression in Aeolian Landforms and Sediments Earth Science Reviews 12: 279-310 Bowler JM, Kotsonis A, Lawrence CR (2006) Environmental Evolution of the Mallee Region, Western Murray Basin Proceedings of the Royal Society of Victoria: 161-210
McLaren S, Wallace MW, Pillans BJ, Gallagher SJ, Miranda JA, Warne MT (2009) Revised stratigraphy of the Blanchetown Clay, Murray Basin: age constraints on the evolution of paleo Lake Bungunnia Australian Journal of Earth Sciences 56: 259-270
Stephenson AE (1986) Lake Bungunnia - A Plio-Pleistocene megalake in southern Australia Palaeogeography, Palaeoclimatology, Palaeoecology 57: 137-156
Zhiseng A, Bowler JM, Opdyke ND, Macumber PG, Firman JB (1986) Palaeomagnetic Stratigraphy of Lake Bungunnia: Plio-Pleistocene Precursor of Aridity in the Murray Basin, Southeastern Australia Palaeogeography, Palaeoclimatology, Palaeoecology 54: 219-239